Microwave systems for medical hyperthermia, thermotherapy and diagnosis

ABSTRACT

Medical systems and instruments which utilize microwave energy to provide heat treatment and diagnostic imaging of tissue are provided. One medical instrument has a collinear array having antennas configured to transmit electromagnetic energy so that the transmitted energy, in aggregate, is directed toward a desired area of the tissue. Another microwave medical system includes a catheter having an inflatable balloon in fluid communication with an inflation source, and an antenna having a radiating element positioned within the inflatable balloon to transmit electromagnetic energy toward the desired area of tissue. Still another medical instrument includes an antenna having receiving elements for receiving electromagnetic energy radiated from an electromagnetic source, rectifying devices, each having a first end electrically connected to a corresponding one of the receiving elements; and a transmission line electrically connected to a second end of each of the rectifying devices.

BACKGROUND OF THE INVENTION

The present invention relates to microwave devices used in medicalhyperthermia and thermotherapy (referred to collectively herein as "heattherapies"), and diagnostics, and to methods of using such devices.

Localized heat therapies, i.e., hyperthermia (heating to temperaturesbelow 45° C.) and thermotherapy (heating to temperatures above 45° C.),have been intensively investigated for the last two decades for manydisease processes including benign prostatic hyperplasia (BPH) andneoplasms.

However, methods of delivering heat including warm fluid, focusedultrasound, radio frequency, and microwave approaches have been appliedto abnormal tissue with only limited success. The prostate gland is oneorgan targeted as a candidate for applying heat delivery techniques.Because microwave energy can be applied without incision, this approachis one being evaluated. Furthermore, this technique advantageously canbe applied in an outpatient setting.

For heat therapy to be applied safely, it is very important that theapplied heat be confined to the target area (e.g., BPH tumor) alone, toavoid damaging nearby healthy tissue or organs.

Some devices for heat therapy have utilized microwave heating, forexample those disclosed in U.S. Pat. Nos. 4,700,716 and 4,776,086, thedisclosures of which are incorporated herein by reference. Microwaveenergy elevates temperature by increasing the molecular motion withincell structures. As the frequency decreases, tissue penetrationincreases. Small diameter microwave antenna probes have been insertedinto the body through normal body passages or, on occasion, directlyinto diseased tissue, using hollow plastic catheters.

SUMMARY OF THE INVENTION

The invention features medical instruments which utilize microwaveenergy to provide heat treatment and diagnostic imaging of tissue. Theterm "microwave", as used herein, refers to electromagnetic energy inthe microwave frequency spectrum of 300 MHz to 300 GHz.

In one aspect of the invention, the medical instrument includes acollinear array having antennas, each of which is spaced from anadjacent antenna along a longitudinal axis of the array and configuredto transmit electromagnetic energy so that the transmitted energy, inaggregate, is directed toward a desired area of the tissue. The medicalinstrument includes an electromagnetic energy source electricallycoupled to the collinear array to provide the electromagnetic energyindividually to each of the antennas at a selected amplitude and phasecharacteristic.

The invention has numerous advantages. For example, the antennas ofthese medical instruments are arranged, in-vivo, to precisely focus thedirection of transmitted electromagnetic energy (e.g., microwaveenergy), thus allowing well-controlled heating of a target area withoutsubstantially heating adjacent non-target areas (e.g., adjacent vitalorgans or healthy tissue). Because the direction of the applied energyis well-controlled, these instruments require relatively lower amountsof power, e.g., less than 25 Watts, to deliver a high degree of thermaldosimetry, thus ensuring full thermal coverage with a sharp cut-off ofthe thermal envelope. Because the antennas are arranged in an array, thepower deposition pattern of the energy is allowed to be varied byadjusting the voltage amplitude, and phase of the signal supplied toeach antenna, thereby effectively beam steering the electromagneticfield to desired areas.

Embodiments of this aspect of the invention may include one or more ofthe following features.

Each antenna of the collinear array includes a radiating element at thedistal end of the array and a coaxial transmission line extending fromthe radiating element to the proximal end of the array. The coaxialtransmission line connects each antenna to the electromagnetic source.

Each antenna includes an impedance matching network coupled between theradiating element and the coaxial transmission line. The impedancematching network includes a conductive shield surrounding the outerconductor of the coaxial transmission line, the conductive shield havinga first end electrically connected to the outer conductor and a secondend spaced from the first by a distance approximately one-quarterwavelength at the frequency of the transmitted electromagnetic energy.

The medical instrument further includes directing elements positionedadjacent to the radiating elements for enhancing beam steering. Thesedirecting elements include an elongated dielectric member extendingalong the longitudinal axis of the array and having a distal end with anelectrically conductive coating.

The medical instrument further includes a temperature detector forsensing the temperature at a location proximate to one of the radiatingelements and, in response to the sensed temperature, provides signals tothe electromagnetic energy source to control the amplitude and phasecharacteristics of the energy provided to that one of the radiatingelements. In certain embodiments, each antenna has an associatedtemperature detector.

The medical instrument further includes a catheter having a proximalend, distal end, and an inner lumen extending therebetween, the cathetersized to receive the collinear array. The catheter includes a balloon,at its distal end, formed of a material transmissive of theelectromagnetic energy.

The medical instrument further includes a microwave heat pipe having adistal end for withdrawing heat from the target area. The heat pipeincludes a conduit having a proximal end connected to a source ofcoolant (e.g., a condenser) and an enlarged distal portion at the targetarea. In certain embodiments, a plurality of microwave heat pipes areprovided, each associated with a corresponding one of the antennas.

The electromagnetic energy has a frequency in a range between 0.3 and 10GHz and a power level in a range between about 100 mwatts and 150 watts.

In a related aspect of the invention, a medical heat treatment systemincludes a pair of medical instruments, each including an antenna systemdisposed within a catheter, with at least one of the antenna systemsbeing in the form of a collinear array of antennas extending through aninner lumen and positioned at the distal end of an associated one of thecatheters. The catheters are of the type having a proximal end, distalend, and a longitudinal axis extending therebetween. The treatmentsystem also includes an electromagnetic energy source electricallycoupled to said collinear array to provide the electromagnetic energyindividually to each of said plurality of antennas at a selectedamplitude and phase characteristic.

In a related aspect of the invention, a method of treating the prostatewith the medical treatment system described above, includes thefollowing steps. A first one of the pair of medical instruments ispositioned within the urethra. A second one of the pair of medicalinstrument is positioned within the rectum. Electromagnetic energy isapplied to the first of the pair of medical instruments with the secondone of the pair of medical instruments receiving the energy afterpassing through the prostate.

In embodiments of these aspects of the invention, the antenna systemthat is in the form of the collinear array of antennas is a transmittingantenna system, with the other antenna system being a receiving antennasystem.

The treatment system also includes a network analyzer (e.g., scalar orvector) connected to the receiving antenna system, the receiving antennasystem receiving signals from the transmitting antenna system. Thesignals are representative of the material properties of the mediapositioned between the receiving antenna system and the transmittingantenna system. For example, the signals can be representative of theimpedance of the media. Alternatively, the signals can represent theattenuation constant of the media.

In another aspect of the invention, a microwave medical system includesa catheter having an inflatable balloon in fluid communication with aninflation source, and an antenna configured to extend through an innerlumen of the catheter and having a radiating element positioned withinthe inflatable balloon to transmit electromagnetic energy toward adesired area of tissue.

By positioning the antenna within the inflatable balloon and controllingthe level of inflation, the directivity and gain characteristics of theantenna can be advantageously controlled. For example, when the balloonis fully inflated, the antenna is configured to provide a highlyfocussed, highly directive radiation pattern. Changing the volume offluid introduced into the balloon changes its dimensions (e.g.,diameter). Thus, the radiation pattern of antenna can be altered ormodulated by the therapist by inflating and deflating the balloon. Thefluid introduced into the balloon can serve as a heat sink for coolingthe antenna positioned therein.

Embodiments of this aspect of the invention may include one or more ofthe following features.

The inflatable balloon includes a surface (preferably the inner surface)having an electrically conductive member formed thereon for directingelectromagnetic radiation transmitted by the antenna toward the desiredarea of tissue. The electrically conductive member may be in the form ofa sheet formed on the inner surface of the inflatable balloon or,alternatively, a wire positioned adjacent to the inner surface of theinflatable balloon.

The balloon includes a second electrically conductive member with thefirst-mentioned electrically conductive member positioned to reflect theelectromagnetic energy directed away from the desired area of tissueback toward the tissue and the second electrically conductive memberpositioned between the antenna and the desired area of tissue. Thus,inflation and deflation of the balloon changes the relative spacingbetween the antenna and the reflecting and directing conductive members,respectively.

The catheter further includes a second inflatable balloon, which in aninflated condition fixably positions the catheter within a body lumen.This second inflatable balloon, in essence, serves the mechanicalfunction of retaining the catheter in a fixed position, while the firstinflatable balloon relates to improving the electromagnetic function ofthe antenna. The catheter includes a first inflation lumen for providinginflation fluid to the first one of the inflatable balloons and a secondinflation lumen, separate from the first inflation lumen, for providinginflation fluid to the second one of the inflatable balloons.

The antenna system includes a first radiating element (e.g., a dipole)having a longitudinal antenna axis and a proximal end coupled to anelectromagnetic source; and a second radiating element, in operation,electrically floating relative to electrical ground. The secondradiating element includes a conductor helically wound about thelongitudinal antenna axis and coaxially positioned around the firstradiating element to receive energy radiated by the first radiatingelement, and an impedance element (e.g., a capacitor) electricallyconnected between preselected windings of the helically wound secondradiating element.

By electrically connecting impedance elements between windings of theradiating element, an antenna system of this configuration is sized tobe significantly smaller than a one-half wavelength radiating structurewhile providing a current distribution similar to that of a one-halfwavelength radiating structure.

The first radiating element is moveable with respect to the secondradiating element along the longitudinal antenna axis. Thus, thetherapist has the ability to optimize the impedance match and,therefore, maximize power transfer from the source to the antenna.Certain embodiments may further include a device for measuring an inputimpedance characteristic (e.g., reflection coefficient) of the firstradiating element, as well as a network analyzer connected to thereceiving antenna system. The receiving antenna system receives signalsfrom the transmitting antenna system, said signals representative of thematerial properties, such as impedance, of the media positioned betweenthe receiving antenna system and the transmitting antenna system.

In still another aspect of the invention, a medical device includes anantenna having receiving elements for receiving electromagnetic energyradiated from an electromagnetic source; rectifying devices, each havinga first end electrically connected to a corresponding one of thereceiving elements; and a transmission line electrically connected to asecond end of each of the rectifying devices.

In essence, the antenna includes a number of rectifying elements (e.g.,diodes) having first ends connected to a corresponding receiving element(which may be the leads of the diodes) and second ends connected to acommon transmission line. Thus, the rectified outputs for eachrectifying device are combined to provide an aggregate rectified currentfrom all of the rectifying devices. The level of current provided by thediodes using this approach is sufficiently high to allow the antenna tobe directly connected to many conventional power meters without the needfor power amplifiers.

Embodiments of this aspect of the invention may include one or more ofthe following features. Each of the receiving elements are substantiallyparallel with the longitudinal axis of the antenna, andcircumferentially spaced from each other in a plane transverse to thelongitudinal axis of the antenna. The antenna, therefore is far lesssensitive to its radial orientation relative to the longitudinal axis ofthe antenna.

The microwave medical system further includes an impedance matchingnetwork coupled between each of the receiving elements and thetransmission line. The transmission line is preferably in the form of acoaxial transmission line having a center conductor, an outer conductor,and a dielectric member positioned between the center conductor and theouter conductor.

The microwave medical system includes a catheter sized to receive theantenna and having a temperature detector for sensing the temperature ata location proximate to the receiving elements.

Other features and advantages of the invention will be apparent from thedrawings, the following Detailed Description, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic side view of a microwave antenna deviceaccording to one embodiment of the invention, deployed in a ballooncatheter. Transmission lines are shown in phantom lines.

FIG. 2 is a diagrammatic side view of the antennas used in the device ofFIG. 1. FIG. 2A is a diagrammatic side view of the antennas of FIG. 2,showing the radiation pattern obtained during use.

FIG. 3 is a schematic diagram showing the electronics used in amicrowave source suitable for use in the device of FIG. 1.

FIG. 4 is a cross-sectional side view of an antenna arrangement suitablefor use in the device of FIG. 1, in which one of the antennas alsoserves as a heat pipe.

FIG. 5 is a perspective view of an alternative embodiment of an antenna.

FIG. 6 is a cross-sectional side view of the antenna shown in FIG. 5.

FIG. 7 is a cross-sectional view of the proximal end of the antennaalong lines 7--7 of FIG. 6.

FIG. 8 is a side view of the magnetic dipole of the antenna shown inFIG. 5.

FIGS. 9A and 9B illustrate alternative embodiments of exciter dipolessuitable for use with the antenna shown in FIG. 5.

FIG. 10A is a cross-sectional view, partially in schematic form, of asurvey microwave antenna system.

FIG. 10B is a cross-sectional view, partially in schematic form, of thedistal end of the survey microwave antenna system of FIG. 10A.

FIG. 10C is an schematic end view representation of the distal end ofthe survey microwave antenna system of FIG. 10A.

FIG. 11 is a cross-sectional side view of an alternative embodiment of amicrowave medical system for treatment and diagnosis of tissue.

FIG. 12 is a cross-sectional view of the prostate balloon portion of thesystem shown in FIG. 11 in an deflated condition.

FIG. 13 is a cross-sectional view of the prostate balloon portion of thesystem shown in FIG. 11 in an inflated condition.

FIG. 14 is a cross-sectional view of the bladder balloon portion of thesystem shown in FIG. 11 in an inflated condition.

FIG. 15 is a highly diagrammatic view of the male urinary tract,illustrating the use of two microwave antenna devices according to theinvention to image and/or heat the prostate gland.

FIGS. 16A and 16B are cross-sectional and side views, respectively,illustrating the use of two microwave antenna devices for imaging and/orheating the prostate gland.

DETAILED DESCRIPTION

Referring to FIG. 1, microwave antenna system 10 includes a collinearantenna array 12 deployed within a catheter 14. Array 12 is configuredto more precisely focus the direction and level of electromagneticenergy radiating from the array, thereby providing well-controlledheating of the targeted area. Catheter 14 includes a balloon portion 16,mounted at the end of a tube 18, defining an inner lumen 20, and isconstructed to be inserted into a portion of the body, typically througha body opening or passage. Antenna array 12 includes three antennas 22,24, 26, shown in further detail in FIG. 2 and described below.

Antennas 22, 24 and 26 are connected via coaxial transmission lines 28,30 and 32, respectively, to a power system S1 which generates microwaveenergy. A preferred microwave system S1 is shown in FIG. 3 and discussedbelow. Electrical signals representative of the temperature measured bysensors 29, 31, 33, and 35 are received and processed by a temperaturecontrol unit 52 which generates a control signal to the microwave powersystem S1. In response to this control signal, microwave power systemincreases or reduces power delivered to each antenna 22, 24, 26 or array12. As will be discussed below with reference to FIG. 4, in certainembodiments, a heat pipe S3 is connected to antennas 22, 24, 26 tofurther control the precise temperature at the device/tissue interface.A computer 5 is connected via a bus 7 to microwave power system S1,temperature control unit S2 and heat pipe system S3. A computer programis stored on computer 5 and, in response to the signals representativeof power and temperature, controls power S1, temperature control S2, andheat pipe system S3.

As shown in FIGS. 2 and 2a, antenna array 12 includes, in addition toantennas 22, 24, 26, an RF reflector 34 and three RF directors 36, 38and 40. RF reflector 34 and RF director 40 are provided at the end ofdielectric members 42, 44 (dielectric wires or tubes), while the otherRF directors extend from members attached to distal ends of antennas 24and 26 on the same coaxial line. The RF reflector and directors areconstructed by forming a metallic coating on the dielectric wire ortube. The reflectors and directors serve to further improve thedirectivity and gain of antennas 22, 24, 26. For example, reflector 34is positioned behind antenna 22 while director 36 is positioned in frontof the antenna to form a three element Yagi array. The lengths ofreflector 34 is generally commensurate with the length of the antennaswhile the lengths of director 36 is generally shorter (e.g., 75% ofantenna length.)

Temperature sensors are positioned at various points within antennaarray 12. In particular, sensor 29 is positioned at the distal end,sensor 31 at the proximal end, sensor 33 at the center, and sensor 35along a wall of the antenna array to be positioned at the rectal wallopposite the urethral sensors 29, 31, 33, and 35 may be in the form offiber optic sensors surrounded by a dielectric outer envelope. Oneexample of a fiber optic sensor of this type is described in U.S. Pat.No. 4,700,716.

Antenna array 12, as well as the RF reflectors and directors shown inFIG. 2, are fixed in position by potting the array in a solid materialwithin a tube, for example, by placing the array in a tube and fillingthe tube with liquid, hardenable TEFLON® polymer. The tube containingthe array can then be easily inserted into the catheter 14 for use by aphysician.

As shown in FIG. 2A, each of antennas 22, 24, 26 represent individualradiating elements suitably spaced with respect to one another along alongitudinal axis 43 of catheter 14 to form the collinear array. Inpreferred embodiments, each antenna is spaced from an adjacent antennaby one-quarter wavelength (λ/4), approximately 1.115 cm at 915 MHz (intissue with high water content). Although other forms of antennas couldbe used, in this embodiment, antennas 22, 24, 26 are dipole antennas.The relative amplitude and phase of electrical signals provided to eachantenna from microwave system S1 are controlled to obtain a resultantradiation pattern which is the product of the superposition of theradiation patterns from each antenna. In essence, each antenna isindependently controlled so that their respective electric fieldsconstructively add within, and destructively subtract outside, thetarget area. Because the relative amplitude and phase is controlledelectronically by microwave system S1, linear array 12 is said to be anelectronically-scanned array. With this approach, a radiation patternwith a desired narrow beamwidth and direction provides relatively hightemperature and focused heating to the target area.

Furthermore, by varying the relative amplitude and phase of theelectrical signals provided to each antenna 22, 24, 26, a radiationpattern can be generated over a relatively broad range. For example, asshown in FIG. 2A, radiation pattern 41 is shown being swept betweenpositions 41a, 41b, and 41c.

To illustrate the improvement achieved by the collinear arrayarrangement, calculations were made at 915 MHz with antennas 22, 26 inphase opposition to antenna 24. The half power beam width (HPBW) wasmeasured to be 20°, as compared to 45° for a single dipole. A furtheradvantage observed during these measurements was that sidelobes of theresultant radiation pattern were suppressed significantly in lossy media(e.g., tissue with high water content), relative to that observed from asingle dipole. This significantly narrower beamwidth allows the usergreater flexibility in steering the beam, thereby controlling heating ofmaterial.

As shown in FIG. 3, a preferred microwave power system S1 includes fouroutput ports 80, 82, 84, 86, coupled to a four-channel microwave powersource 88 capable of providing approximately 12 watts of continuous wavepower at 915 MHz to individual ones of antennas 28, 30, 32 of antennaarray 12. Note that in this embodiment, because array 12 only includesthree antennas, an extra port is available in the event that one of theports malfunctions. Each port is coupled to a respective output ofsource 88 through individual bi-directional couplers 90. A fraction(e.g., 20 dB) of the microwave power source 80 is tapped from couplers90 and provided to a vector voltmeter 92 through a sequence of rotaryswitches 94, 96, 98. A switch controller 100 is used to select one ofports 80, 82, 84, 86 being examined at any given time. A 30 Dbattenuator is connected at the output of rotary switch 98 to protectvector voltmeter 92 from excessive power levels. As stated above,computer 5 is used to control the components of system S1 including,power source 80, vector voltmeter 92 and switch controller 100, via bus7.

Although not necessary for achieving super-directive radiation patterns,in certain embodiments, each antenna 22, 24, 26 of array 12 can alsoserve as a "heat pipe". The heat pipe serves as a source or sink forthermal energy at a desired area, so that even greater control oftemperature at the interface of the heat pipe and adjacent material isachieved. It is important to recognize that although the device iscalled a "heat pipe", in operation, it can provide both heating as wellas cooling, depending on whether the fluid (e.g., liquid or gas) is hotor cold.

Referring to FIG. 4, for purposes of illustration, antenna 22 of array12 is shown having the structure for providing heat pipe temperaturecontrol. Heat pipe 50 includes an antenna portion/cooling region 52, aheat exchanger 56 and a flexible RF coaxial transmission line 58connecting the antenna portion 52 to microwave power source S1. Theantenna portion 52 is formed by a hollow conductive pipe 60 and adielectric sheath 70 extending substantially the entire length of theconductive pipe. As described above in conjunction with FIGS. 1, 2, 2A,conductive pipe 60 is one part of coaxial transmission line 28 fortransmitting energy from source S1 to antenna portion/cooling region 52.When used as a heat pipe, conductive pipe also functions as a capillarywick for a liquid or gas 62 passing therethrough. The capillary actionis accomplished by having a relatively larger diameter portion 66 at theantenna portion, to provide evaporative cooling, and a relativelysmaller diameter "wick" portion 67 extending between portion 66 and heatexchanger 56. Larger diameter portion 66 is approximately λ/2 in length.At a junction 71, wick portion 67 extends beyond transmission line 58 tothe heat exchanger 56 in the form of a dielectric tube 69.

When used in applications where cooling is required, heat exchanger 56acts as a condenser having a refrigerant (e.g., cryogenic fluid). Apressure mechanism 57 under the control of computer 5 is used to controlthe amount and rate at which fluid is delivered to cooling region 52. Asdiscussed above, in response to electrical signals from temperaturecontrol unit S2, computer 5 controls microwave system S1 to generateelectrical signals with the appropriate amplitude and phasecharacteristics for providing a focused beam in the direction of thetarget area. In embodiments having a heat pipe 50, computer 5 alsocontrols heat exchanger S3 to convey cooling fluid within antennaportion/cooling region 52 to remove heat, thereby allowing rapid andprecise adjustment of the temperature at the interface between thecooling region and surrounding material.

By constructing one or more of antennas 22, 24, 26 as a heat pipe, therelatively high, and focused heating characteristics provided by eachantenna of array 12 can be controlled with even greater precision, byquickly and reliably delivering coolant or heat to the target area,thereby decreasing or increasing the temperature, respectively, at thetarget area. Further details concerning the thermodynamic operation ofheat pipes suitable for use in antenna array 12 are described in U.S.Pat. No. 5,591,162, entitled "Treatment Method Using a Micro Heat PipeCatheter", which is incorporated herein by reference.

In certain applications, antenna array 12 may include transformers 46,48, positioned between antennas 22, 24 and the microwave power systemS1. These transformers present a well-matched impedance to power systemS1 within a predetermined frequency range. Transformers 46, 48 arespaced from respective antennas 22, 24 by one-quarter wavelength.Transformer 54 is provided by the combination of conductive pipe 60, anouter conductive coaxial sheath 64, dielectric sheath 70, and a metalliccylinder 73. Outer conductive coaxial shield 64 surrounds dielectricsheath 70 and extends along the length of conductive pipe 60 untilterminating at a point just before larger diameter portion 66. Metalliccylinder 73 is approximately one-quarter wavelength in length and coversouter conductive coaxial shield 64, thereby electrically shorting thepair of members at point A. This electrical short presents an effectiveopen circuit (high impedance) along the transmission line one-quarterwavelength away from the short.

Transformer 54 minimizes the reflected power seen by microwave powersource S1. Equally important, transformer 54 also prevents leakage ofantenna currents along the outside structure of array 12. By appropriateselection of operating parameters, transformer 54 can be designed toprovide both a minimum reflection coefficient as well as minimum leakagewithin the same frequency range.

Using transformers 46, 48 is not limited in an antenna array having aheat pipe. Rather, all of the advantages provided by the use of suchtransformers, as described above, are achieved when antenna system 10 ofFIG. 1 does not include heat pipe system S3.

To use microwave antenna system 10, a physician would insert catheter 14into a desired region of a patient's body, using a body passage, such asthe urethra. The physician would then activate the microwave energysource S1 to deliver energy to a target region adjacent to the bodypassage. During heating, computer 5 monitors the information collectedby temperature control unit S2 and adjusts the amount of energydelivered by microwave power source S1 accordingly. In embodiments whichinclude a heat pipe, computer 5 also controls the delivery of the fluidto the surgical site, such as, by providing appropriate control signalsto pressure mechanism 57. The rate of heat delivered is matched to thethermal conductivity of the tissue and the degree to which the tissue isperfused.

Referring to FIG. 5, another embodiment of an antenna 200 well-suitedfor use within antenna array 12 is shown. It is important to note thatalthough only one antenna is shown, multiple antennas can be extendedthrough a catheter. Antenna 200 includes a pair of radiating elements,one of which serves as a movable exciter dipole 202, the other whichserves a magnetic dipole element 204. This configuration allows thesurgeon to adjust the position of exciter dipole 202 relative tomagnetic dipole element 204 so that both elements radiate together withnear-perfect impedance match, thereby maximizing power transferefficiency to the surrounding tissue. As was the case with theembodiment shown in FIGS. 1-4, antenna 200 is positioned alone or withlike antennas within a catheter 205 having an inflatable balloon portion203.

Referring to FIGS. 6 and 7, exciter dipole 202 includes a micro-coaxialtransmission line 206 which extends from a proximal end 208 connected toa corresponding port of power source S1 to a center-fed dipole element207. Center-fed dipole 207 is defined by a gap 209 formed by removing aportion of outer conductor 210 at a distance one-quarter wavelength (atthe desired frequency of operation) from a distal end 211 of antenna200.

Transmission line 206 includes a center conductor 208 spaced from anouter conductor 210 by dielectric 212 to provide a transmission linewith a characteristic impedance of 50Ω. Exciter dipole 202 also includesa bifurcated impedance transformer 214 defined by a conductive shield216, which extends along a portion of transmission line 206. Conductiveshield 216, which may be braided or in the form of a solid member, isdisposed around and spaced from outer conductor 210 of transmission line206 by a dielectric layer 218. Impedance transformer 214 ensures a goodimpedance match between center-fed dipole element 207 of exciter dipole202 and transmission line 206 (50Ω.) A more complete description of theconstruction and theoretical operation of a similar impedancetransformer and its application within a medical instrument can be foundin U.S. Pat. No. 4,776,086, entitled "Method and Apparatus forHyperthermia Treatment", which is incorporated herein by reference.

Referring to FIG. 8, magnetic dipole 204 is in the form of a helicalwinding 213 which, in this embodiment, has 21 turns wound about alongitudinal axis 220 of element 204 and has an inner diameter slightlylarger than the outer diameter of center-fed dipole 207. Helical antennastructures similar to helical winding 213 are described in Chapter 7"The Helical Antenna" of Antennas by J. D. Kraus McGraw Hill PublishingCo. (1988), which is incorporated herein by reference. The effects ofimpedance loading on helical wound antennas is described in Chapter 2"Wire Antennas" of Small Antennas by K. Fujimoto et al., ResearchStudies Press Ltd. (1987), which is incorporated herein by reference.U.S. Pat. No. 5,755,754, entitled "Device and Method for AsymmetricalThermal Therapy with Helical Dipole Microwave Antenna", describes anapproach for using a helical antenna to thermally treat tissue and isalso incorporated herein by reference.

Capacitors 222 are electrically connected between predetermined ones ofthe turns of helical winding 213. Although helical long wire 213 has alength (L) which is significantly less than one-half wavelength, properpositioning of capacitors 222 along the length of helical winding 213provides a current distribution resembling a one-half wavelengthradiating structure. Without impedance loading, a helical winding ofmuch longer length would be required for resonance and efficientradiation at the desired frequency of operation.

As stated above, because exciter dipole 202 is movable within magneticdipole 204, the surgeon can axially position exciter dipole 202 tooptimize the impedance match between the elements, thereby maximizingmicrowave energy transfer to the magnetic dipole and, in turn, to thesurgical site desired to be heated. Axial movement is critical becausethe dielectric properties of the tissue itself changes as it is heated,thus causing a change in its impedance characteristics. Thus, theoptimum position of exciter dipole 202 relative to magnetic dipole 204is likely to change as the temperature of the tissue changes. In use,the surgeon adjusts the axial position of exciter dipole using a precisemechanical control mechanism, such as a micrometer 224 (FIG. 5), as hemonitors an indicator 226 (FIG. 5) showing the quality of impedancematch (e.g., reflection coefficient indicator.) It is important to notethat movement of exciter dipole 202 within magnetic dipole 204 candramatically change the magnitude of the reactance relative to theimpedance (e.g., 50Ω) of center-fed dipole 207. However, regardless ofthe position and the relative reactance magnitude, the currentdistribution and resulting radiation pattern should be substantially thesame.

Referring to FIGS. 9A and 9B, alternative constructions of an exciterdipole 202a, 202b are shown. In particular, exciter dipole 202a isprovided in the form of a single turn Faraday shielded loop, whileexciter dipole 202b is formed as a multi-turn loop. Exciter dipole 202bmay include additional capacitive loading elements connected between oneor more loops.

From an electromagnetic wave standpoint, magnetic dipole 204 is floating(i.e., it has no ground plane) and is excited in the T₀ mode by exciterdipole 202. Excitation in this manner is similar to exciting arectangular waveguide in the TE₁₀ mode with an electric monopolepositioned along the center line of a broad wall of the waveguide.

In use, antenna 200 is introduced to the surgical site through catheter205. Electrical power is applied to exciter dipole 202 from power sourceS1. By observing the amount of reflected power on indicator 126, thesurgeon adjusts the position of exciter dipole 202 within magneticdipole 204 using micrometer 224. When the level of reflected power is ata minimum the surgeon is assured that he has found the optimum position.

Referring to FIGS. 10A-10C, a receiving antenna 250 for detecting energyradiated from, for example, antenna 200 is shown. Receiving antenna 250includes a diode assembly 252 positioned at the distal end of receivingantenna 250. Diode assembly 252 includes rectifying elements in the formof diodes 254, which have their cathodes 256 connected at a commondistal node 258. Anodes 260 of the diodes 254 are connected to one ofleads 262, which serve as elements for receiving and conveying theelectromagnetic wave energy to the diodes. The opposite ends of leads262 are connected to an outer conductor 268 of a micro-coax transmissionline 266 through a conductive washer 270. Each cathode of diodes 254 isconnected to a center conductor 264 of micro-coax transmission line 266.Transmission line 266 is of the same construction of transmission line206 of antenna 200 (see FIG. 6). Specifically, transmission line 266includes outer conductor 268 spaced from center conductor 264 bydielectric (not shown) to provide a 50Ω characteristic impedance. Abifurcated impedance transformer 272 defined by a conductive shield 274extends along a portion of transmission line 266. Conductive shield 274,which may be braided or in the form of a solid member, is disposedaround and spaced from outer conductor 268 of transmission line 266 by adielectric layer 278. Impedance transformer 272 ensures a good impedancematch between diode assembly 252 and transmission line 266.

Each diode 254 rectifies the electromagnetic waves received along itsassociated lead 262 and produces a direct current (DC) signal. Thecurrent generated by each diode 254 is summed at node 258 and carried toa measurement system (not shown) via coaxial transmission line 266.Diodes 254 may be encapsulated or potted to lend mechanical support toassembly 252.

Referring to FIGS. 11-14, a microwave medical system 300 particularlywell-suited for use with antenna 200 is shown. System 300 includes acatheter 302 having an inflatable yagi balloon 304 and an inflatablefixation balloon 306. As will be discussed in greater detail belowfixation balloon 306, in operation, is used to mechanically fix theposition of the catheter within a body passage, such as the urethra.When positioned in the rectum, a rectal catheter can be fixed inposition by external means. On the other hand, yagi balloon 304 is usedto control the delivery of energy radiated from antenna 200 tosurrounding tissue. In particular, by varying the amount of fluid (e.g.,water) and thus, the amount of dielectric material between the radiatingantenna and the tissue, the radiation pattern of the energy from antennais controlled. The fluid can also serve as a heat sink medium forwithdrawing heat away from antenna. Indeed, providing additives to thefluid or using a different fluid (e.g., saline) can enhance the heatsinking effect.

In certain applications, the temperature of the fluid or the dielectricconstant of the fluid can be controlled to increase the efficacy of thetreatment. For example, by changing the salinity of water used toinflate yagi balloon 304, the dielectric constant can be modulated.

In this embodiment, yagi balloon 304 expands symmetrically. However, incertain applications, the balloon can be constructed to expandasymmetrically, for example, with a spacing between antenna 200 anddirector 330 greater than that between the antenna and reflector 328.

Catheter 302 includes a central passage 308 which is sized to allowantenna 200 to extend to yagi balloon 304. In certain applications,central passage 308 may also be used for passing catheter 302 over apositioning stylet (not shown). A locking mechanism 310 for fixing theposition of antenna 200 relative to yagi balloon 304 is provided at theproximal end of catheter 302. A fluid insertion chamber 312 and a fluidextraction chamber 314 surround central passage 308 for allowing coolingfluid to be introduced and withdrawn, respectively, from catheter 302 inthe area of yagi balloon 306 during operation of antenna 200.

A lumen 316 extends through catheter 302 from yagi balloon 304 to asyringe valve 318, which is connected to a fluid source (e.g., syringe)for inflating the yagi balloon. A second lumen 320 similarly extendsthrough catheter 302 from fixation balloon 306 to a syringe valve 322,which is connected to a separate fluid source (e.g., syringe) forinflating the fixation balloon. Temperature sensors 324 are attached toan outer surface of catheter 302 and are electrically connected totemperature control unit S2 (FIG. 1) via fiber optic lines (not shown)positioned through lumens 326 extending through the catheter to providesignals indicative of the temperature of the tissue.

As was the case with the embodiment of array 12 shown in FIGS. 1-4,reflector and director elements can be used to further enhance focusingof radiated energy from antenna 200 to a particular area of tissue.

Referring in particular to FIG. 12, in one embodiment, one or morereflectors 328 can be formed along inner surface 329 of yagi balloon 304to direct any radiated energy incident onto the reflector back towardthe desired tissue area. In this embodiment, reflector 328 is in theform of a thin conductive sheet covering an angular area of about 60°.In addition to reflector 328, a director 330 in the form of a conductivesheet is formed on a portion of inner surface 329 diametrically oppositethat of reflector 328. Director 330 covers an area of 30°. Inalternative embodiments, reflector 328 and director 330 can be in theform of a conductive mesh or set of wires. Changing the volume of fluidwithin yagi balloon 304, changes the balloon diameter, as well as therelative spacing between antenna 200 and reflector 328 and director 330.

This arrangement of positioning the active antenna element 200 between areflector 328 and a director 330 provides, in essence, an antenna withincreased directivity and higher antenna gain, commonly associated withYagi antennas. This increased gain characteristic, which can be as muchas 6 Db, advantageously allows the required power to antenna 200 to bereduced by a factor of four. Operating at reduced power, allows lowerpower, less expensive power sources to be used, increases reliability ofthe source, and provides a significantly safer medical procedure.Furthermore, where higher power is available from the source and isdesired for heating, the increased gain characteristic of antenna 200allows for deeper penetration of heat in tissue (e.g., prostate.)

As shown in FIG. 15, microwave antenna system 300 is particularlyattractive for use in the treatment and diagnosis of prostatic cancer aswell as benign prostatic hyperplasia (BPH). For example, cancer of aprostate 101 often originates on a posterior portion of the prostateclose to the rectal wall 102. Thus, system 300 is useful for thistreatment because access to prostate 101 can be achieved through therectum 104 and/or the urethra 106. For example, the physician may insertmicrowave antenna system 300 within the urethra 106 while positioningreceiving antenna 50 through the anus 108 and into rectum 104, as shown.In this application microwave antenna system 300 is used to achieve ahigh degree of heat uniformity through prostate 101, while receivingantenna 250 monitors the level of energy radiated by antenna system 300.

One approach for treating or diagnosing the prostate using these devicesfollows. Catheter 302 is first introduced within the urethra andappropriately positioned using well-known positioning techniques, suchas ultrasound or more radiopaque markers on catheter 302, so that yagiballoon 304 is positioned adjacent prostate 101. Once positioned, thetherapist or surgeon introduces fluid through valve 322 to inflatefixation balloon 306, thereby fixing the position of catheter 302 withinthe passage.

Antenna 200 is then introduced through central lumen 308 until magneticdipole 204 and center-fed dipole 207 are both positioned within yagiballoon 304. A relatively low level of power (e.g., 100 mwatts) is thenapplied to antenna from power source S1. While observing reflectioncoefficient indicator 226 (FIG. 5), the axial position of exciter dipole202 is adjusted relative to magnetic dipole 204 until a minimumreflection coefficient is achieved, thereby ensuring maximumtransmission power into prostate. The applied power from power source isincreased (e.g. 1 to 2 watts) and fluid is then introduced into yagiballoon 304 via valve 318 so that the yagi balloon inflates.

Receiving antenna 250 is introduced within the rectum at a positionclose to the prostate to detect energy radiated by antenna 200positioned within urethra 104. Thus, any changes in the radiationpattern of antenna 200 caused by volume of fluid changes in yagi balloon304 can be detected by receiving antenna 250 and observed, for example,on display monitor 5a. Thus, the radiation pattern of antenna 200 can bealtered or modulated by the therapist. In other applications, the levelof power applied to antenna 200 from the source can be modulated tocontrol heating of the tissue.

As was stated above, the dielectric constant of the radiated tissuechanges due to heating primarily because the amount of fluid in thetissue changes. Thus, it may be desirable during the procedure for thetherapist to readjust the axial position of exciter dipole 202 relativeto magnetic dipole 204 once again to obtain a minimum reflectioncoefficient.

Referring to FIGS. 16A and 16B, a diagnostic approach for usingmicrowave antenna system 300 for treating prostate 101 is shown. In thisapproach, antenna system 300 is used in a diagnostic mode to locatetissue boundaries, created by the inherent dielectric contrast betweenabnormal and normal tissues by virtue of their relative water contents.

In this diagnostic mode, microwave antenna system 300 of the type shownin FIG. 11 is passed through urethra 106 while receiving antenna 250 isintroduced into rectum 104. Receiving antenna 250 is used to receivesignals transmitted from antenna system 300. The signals transmittedfrom antenna system 300 are attenuated by the electrical characteristicsof the tissue media. Thus, by measuring certain characteristics of thesignals as they pass through the tissue, certain material properties ofthe tissue, such as the electrical attenuation constant (α) inNepers/length can be determined. The attenuation characteristics of thesignals passing through the tissue provide an indication as to the type(e.g., bone, muscle, tumor) and relative normalcy of that tissue. Forexample, healthy muscle tissue typically has less water content thancancerous tissue. Thus, when the narrow beamwidth energy transmittedfrom antenna system 300 is swept through a region of the healthy tissueand into the neoplastic tissue, as well as through heated and unheatedtissue, a change in the value of the attenuation constant is likely tobe observed.

In the above described procedure, receiving antenna 250 was positionedwithin the rectum to detect radiated energy from microwave antennasystem 300. In other procedures, a microwave antenna system 300 can beinserted in both rectum 104 and urethra 106 so that prostate 101 isradiated from two different positions.

Computer 5 would generally include a computer display monitor 5a(FIG. 1) for displaying continuous readings of temperature changes atboundaries of a simulated target organ (e.g., prostate) illustration oran ultrasound image. A schematic template of the target organrepresenting the anatomy would be displayed with superimposed differentcolors representing different temperature ranges at different regions ofthe organ. Thus, the therapist or surgeon is able to determine, in realtime, the target site and the effectiveness in applying heat from thesystem. The monitor can display the temperature detected by each of thesensors as a function of time and provide beginning and end points forthe treatment.

Based on signals received from the sensors computer 5 is capable ofissuing warning messages to be displayed on the monitor whentemperatures exceed predetermined threshold values. Computer 5 may alsoautomatically shutdown power source S1 if, for example, the temperaturesremain high for an unacceptable time period or if a fault is detected inthe system. Computer 5 also includes memory for storing statistical dataincluding patient information, current laboratory data, as well as alldata collected during the procedure.

An article by McCorkle et al. entitled "Monitoring a Chemical PlumeRemediation via the Radio Imaging Method", which is incorporated byreference, provides a mathematical analysis for determining theelectrical attenuation constant.

The antenna systems described above are well-suited for this applicationbecause both antenna systems 10 and 300 as well as receiving antenna 250can remain stationary with the direction of the beam of energyelectronically swept through various positions 110-114 by varying theamplitude phase and characteristics of the microwave power source S1. Anetwork analyzer 115 (FIG. 15), for example, an HP 8510 Vector NetworkAnalyzer (a product of Hewlett Packard Company, Palo Alto, Calif.) isconnected to antenna system 250 to measure the impedance at the distalend of antenna system 250. The impedance is used to derive theattenuation and phase constant values for each measurement.

It should also be appreciated, however, that a transmitting microwaveantenna can be physically moved, for example, by the physician, toprovide a series of attenuation characteristic values which can be usedto characterize the tissue in the target area. The transmitting antennacan also be rotated about its axis to provide further directionalcontrol of the transmitted beam of energy.

Other embodiments are within the scope of the claims.

It is important to appreciate that catheters 14 and 302 can be any of awide variety of catheters of different configurations and sizes. Theparticular application in which the microwave antenna system is usedwill generally dictate the choice of delivery catheter, stylet, as wellas the number and particular configuration of antennas. For example,when used in the urethra, flexible foley-type catheters ranging in sizebetween 18-28 F can be used. On the other hand, when introduced into therectum larger catheters from 22 to 32 F may be more appropriate. Therectal catheter may be accompanied by an ultrasound imaging transducer,both of which are incorporated in a holding sheath. The catheters mayinclude small protrusions positioned along the length of the cathetersto facilitate their positioning during delivery. The antennas themselvesare radiopaque, as well, to aid in ascertaining their position.

Furthermore, although the above embodiments describe close-endedcatheters, alternative applications may require the use of open-endedcatheters for end-fire configurations. Additional lumens for introducingirrigation fluids or therapeutic agents (e.g., chemotherapeutic agents,hypothermia, and/or thermal sensitizers) can also be deliveredsimultaneously or successively to enhance thermal therapy provided bythe antennas.

The approach described above utilized the electrical attenuationconstant for characterizing tissue. However, other parameters may bederived from the impedance measurements to characterize the tissue aswell. For example, the permittivity or complex dielectric constant(.di-elect cons.*=.di-elect cons.'-j.di-elect cons.") as an indicator ofwater content in tissue, which, as described above, may be used todetermine the type of tissue. With this approach, a calibrationprocedure is generally required to establish impedance reference valuesfor various known materials, ranging from, for example, distilled waterto a sample with no water. Between these two extremes, various types oftissue and neoplasms can be measured with the antenna system toestablish a database of impedance values for different tissue.

The ability to use microwave antenna system 10 in a diagnostic mode is apowerful tool, particularly when the antenna system is also used toprovide hyperthermia treatment (i.e., in a heating mode). In essence,the diagnostic mode is used to identify and isolate areas which requiretreatment in the heating mode. Thus, antenna system 10 provides adynamic, dual-function approach for treating tissue. Use of antennasystem 10 in this manner is particularly important when one recognizesthat the dielectric properties of tissue change with temperature. Byalternating between the heating and diagnostic modes, precise control ofthe level and direction of heat applied by microwave source can beadministered. For example, during heating, the water content of thetissue will decrease and, therefore, the rate at which heat is absorbedby the tissue diminishes. Furthermore, the decrease in water contentcauses the organ to shrink in size. In the diagnostic mode the change insize and water content will be reflected in a change in impedance, aswell as dielectric constant. Based on this change, the amplitude andphase characteristics of the signals applied to each antenna of thearray can be altered to more precisely control the direction and levelof energy applied to the tumor.

As stated above, in some cases, the impedance of the tissue beingtreated may change considerably during treatment. If this occurs, thephysician may remove the catheter and insert a second microwave antennadevice 300 or 10 having different characteristics. For example, amicrowave antenna system having slightly different spacings betweenadjacent antennas may be substituted.

Although, FIGS. 6A and 6B show only a single radiating microwave system10, it should be appreciated that a separate receiving antenna system250 allows the use of two or more radiating microwave antenna systems300 to provide a greater variety of different heating pattern shapes.

For example, while heat pipe S3 has been shown in FIG. 4, and discussedabove, as being part of the antenna array, the heat pipe could beprovided as a separate device. Moreover, the heat pipe may be operatedin such a manner as to iteratively cool and heat the tissue adjacent theantenna.

Also, while FIG. 5 shows the use of a plurality of microwave antennadevices introduced through the urinary bladder and rectum for treatmentof the prostate, similar methods can be used in other areas of the body,for example, the liver or kidney.

Still other embodiments are within the scope of the claims.

What is claimed is:
 1. A medical instrument for treatment of tissue, themedical instrument comprising:a collinear array having a longitudinalaxis and a plurality of antennas, each antenna spaced from an adjacentantenna along the longitudinal axis of the array, each antennaconfigured to transmit electromagnetic energy so that the transmittedenergy, in aggregate, is directed toward a desired area of the tissue;and an electromagnetic energy source electrically coupled to saidcollinear array to provide the electromagnetic energy individually toeach of said plurality of antennas at a selected amplitude and phasecharacteristic.
 2. The medical instrument of claim 1 wherein eachantenna includes a radiating element at a distal end of the array and acoaxial transmission line extending from the radiating element to aproximal end of the array.
 3. The medical instrument of claim 2 whereineach antenna further includes an impedance matching network coupledbetween the radiating element and the coaxial transmission line.
 4. Themedical instrument of claim 3 wherein the coaxial transmission lineincludes a center conductor, an outer conductor, and a dielectric memberpositioned between the center conductor and the outer conductor and theimpedance matching network includes a conductive shield surrounding theouter conductor, the conductor shield having a first end electricallyconnected to the outer conductor and a second end spaced from the firstend by a distance approximately one-quarter wavelength at the frequencyof the transmitted electromagnetic energy.
 5. The medical instrument ofclaim 2 wherein the medical instrument further includes a plurality ofdirecting elements positioned adjacent to the radiating elements toenhance beam steering.
 6. The medical instrument of claim 5 wherein thedirecting elements include an elongated dielectric member extendingalong the longitudinal axis of the array, the dielectric member having adistal end with an electrically conductive coating.
 7. The medicalinstrument of claim 2 further including a temperature detector forsensing the temperature at a location proximate to one of the radiatingelements and, in response to the sensed temperature, providing signalsto the electromagnetic energy source to control the amplitude and phasecharacteristics of the energy provided to that one of the radiatingelements.
 8. The medical instrument of claim 7 further including aplurality of temperature detectors, each associated with a correspondingone of the antennas.
 9. The medical instrument of claim 1 furtherincluding a catheter having a proximal end, a distal end, and an innerlumen extending therebetween, the catheter sized to receive thecollinear array.
 10. The medical instrument of claim 9, wherein thecatheter includes a balloon at its distal end, the balloon formed of amaterial transmissive to the electromagnetic energy.
 11. The medicalinstrument of claim 9 wherein further including a microwave heat pipehaving a distal end for withdrawing heat from a target area of thetissue.
 12. The medical instrument of claim 11 wherein the heat pipeincludes a conduit having a proximal end connected to a source ofcoolant and an enlarged distal portion at the target area.
 13. Themedical instrument of claim 11 wherein the source of coolant includes acondenser.
 14. The medical instrument of claim 11 further including aplurality of microwave heat pipes, each associated with a correspondingone of the antennas.
 15. The medical instrument of claim 1 wherein theelectromagnetic energy has a frequency in a range between 0.3 and 10GHz.
 16. The medical instrument of claim 1 wherein the electromagneticenergy has a power level in a range between about 100 mwatts and 150watts.
 17. A medical treatment system comprising:a pair of medicalinstruments, each including an antenna system disposed within a catheterhaving a proximal end, distal end, and a longitudinal axis extendingtherebetween, the catheter defining an inner lumen extending along thelongitudinal axis and between the proximal end and distal end; at leastone of the antenna systems being in the form of a collinear array ofantennas extending through the inner lumen and positioned at the distalend of an associated one of the catheters; and an electromagnetic energysource electrically coupled to said collinear array to provide theelectromagnetic energy individually to each of said plurality ofantennas at a selected amplitude and phase characteristic.
 18. Themedical treatment system of claim 17 wherein a first one of said antennasystems is a transmitting antenna system, a second one of the antennasystems being a receiving antenna system.
 19. The medical treatmentsystem of claim 17 further comprising a network analyzer connected tothe receiving antenna system, the receiving antenna system receivingsignals from the transmitting antenna system, said signalsrepresentative of the material properties of the media positionedbetween the receiving antenna system and the transmitting antennasystem.
 20. The medical treatment system of claim 19 wherein saidsignals are representative of the impedance of the media.
 21. Themedical treatment system of claim 19 wherein said signals arerepresentative of the attenuation and phase constants of the media. 22.The medical treatment system of claim 17 wherein the electromagneticenergy has a frequency in a range between 0.3 and 10 GHz.
 23. Themedical treatment system of claim 17 wherein the electromagnetic energyhas a power level in a range between about 100 mwatts and 150 watts. 24.A method of treating the prostate with the medical treatment system ofclaim 17, the method including:positioning a first one of the pair ofmedical instruments within the urethra; positioning a second one of thepair of medical instruments within the rectum; and applyingelectromagnetic energy to the prostate with the first of the pair ofmedical instruments.
 25. The method of claim 24 wherein the second oneof the pair of medical instrument is configured to receive theelectromagnetic energy passing through the prostate.
 26. The method ofclaim 24 further comprising applying electromagnetic energy to theprostate with the second of the pair of medical instruments.
 27. Amicrowave medical system comprising:a catheter including:an inner lumenextending along a longitudinal axis between a proximal end and a distalend of the catheter; an inflatable balloon in fluid communication withan inflation source, the inflatable balloon including a surface having afirst electrically conductive member formed thereon for directingelectromagnetic radiation transmitted by the antenna toward the desiredarea of tissue; and an antenna system configured to extend through theinner lumen and having a radiating element positioned within theinflatable balloon, the antenna configured to transmit electromagneticenergy toward a desired area of tissue.
 28. The microwave medical systemof claim 27 wherein the electrically conductive member is positioned onan inner surface of the inflatable balloon.
 29. The microwave medicalsystem of claim 28 wherein the electrically conductive member is in theform of a sheet formed on the inner surface of the inflatable balloon.30. The microwave medical system of claim 28 wherein the electricallyconductive member is in the form of a plurality of wires positionedadjacent to the inner surface of the inflatable balloon.
 31. Themicrowave medical system of claim 27 further comprising a secondelectrically conductive member, the first electrically conductive memberpositioned to reflect the electromagnetic energy directed away from thedesired area of tissue back toward the tissue, the second electricallyconductive member positioned between the antenna and the desired area oftissue.
 32. The microwave medical system of claim 27 wherein thecatheter further includes a second inflatable balloon, which in aninflated condition fixably positions the catheter within a body lumen.33. The microwave medical system of claim 32 wherein the catheterincludes:a first inflation lumen for providing inflation fluid to afirst inflatable balloon; and a second inflation lumen, separate fromthe first inflation lumen, for providing inflation fluid to the secondinflatable balloon.
 34. The microwave medical system of claim 27 whereinsaid antenna system includes:a first radiating element having alongitudinal antenna axis and a proximal end coupled to anelectromagnetic source; and a second radiating element, in operation,electrically floating relative to electrical ground, and including:aconductor helically wound about the longitudinal antenna axis andcoaxially positioned around the first radiating element to receiveenergy radiated by the first radiating element, and an impedance elementelectrically connected between preselected windings of the helicallywound second radiating element.
 35. The microwave medical system ofclaim 34 wherein the first radiating element is moveable with respect tothe second radiating element along the longitudinal antenna axis. 36.The microwave medical system of claim 34 wherein the helically woundsecond radiating element has a first diameter and the first radiatingelement is in the form of a helically wound conductor having a seconddiameter less than the first diameter.
 37. The microwave medical systemof claim 34 wherein the impedance element is a capacitor.
 38. Themicrowave medical system of claim 34 wherein the first radiating elementis a dipole antenna.
 39. The microwave medical system of claim 34further comprising a device for measuring an input impedancecharacteristic of the first radiating element.
 40. The microwave medicalsystem of claim 34 further comprising a network analyzer connected tothe antenna system, the antenna system receiving signals representativeof the material properties of the surrounding tissue.
 41. The microwavemedical system of claim 40 wherein said signals are representative ofthe impedance of the media.
 42. The microwave medical system of claim 34wherein the electromagnetic energy has a frequency in a range between0.3 and 10 GHz.
 43. The microwave medical system of claim 34 wherein theelectromagnetic energy has a power level in a range between about 100mwatts and 150 watts.
 44. A medical device comprising an antenna havinga longitudinal axis and including:a plurality of receiving elements forreceiving electromagnetic energy radiated from an electromagneticsource; a plurality of rectifying devices, each rectifying device havinga first end electrically connected to a corresponding one of theplurality of receiving elements; and a transmission line electricallyconnected to a second end of each of the rectifying devices.
 45. Themicrowave medical system of claim 44 wherein each of the receivingelements are substantially parallel with the longitudinal axis of theantenna, and circumferentially spaced from each other in a planetransverse to the longitudinal axis of the antenna.
 46. The microwavemedical system of claim 44 further comprising an impedance matchingnetwork coupled between each of the receiving elements and thetransmission line.
 47. The microwave medical system of claim 44 whereinthe transmission line is a coaxial transmission line having a centerconductor, an outer conductor, and a dielectric member positionedbetween the center conductor and the outer conductor.
 48. The microwavemedical system of claim 44 wherein each of the rectifying devices is adiode.
 49. The microwave medical system of claim 48 wherein each of thereceiving elements is formed by a lead portion of each of the diodes.50. The microwave medical system of claim 44 further comprising acatheter having a proximal end, a distal end, and an inner lumenextending therebetween, the catheter sized to receive the antenna. 51.The medical instrument of claim 50 wherein the catheter further includesa temperature detector for sensing the temperature at a locationproximate to the receiving elements.
 52. The microwave medical system ofclaim 50 further comprising a network analyzer connected to thetransmission line for receiving rectified signals from the rectifyingelements representative of the material properties of the mediapositioned between the antenna and the electromagnetic source.
 53. Themicrowave medical system of claim 52 wherein the analyzer is configuredto receive signals from the temperature detector representative of thetemperature at a location proximate to the receiving elements.
 54. Themedical instrument of claim 50 wherein the catheter further includes atemperature detector for sensing the temperature at a location proximateto the receiving elements.